Collective search in ants: Movement determines footprints, and footprints influence movement

Collectively searching animals might be expected to coordinate with their groupmates to cover ground more evenly or efficiently than uncoordinated groups. Communication can lead to coordination in many ways. Previous work in ants suggests that chemical ‘footprints’, left behind by individuals as they walk, might serve this function by modulating the movement patterns of following ants. Here, we test this hypothesis by considering the two predictions that, first, ants may turn away from sites with higher footprint concentrations (klinotaxis), or, second, that they may change their turning patterns depending on the presence of footprints (klinokinesis). We tracked 5 whole colonies of Temnothorax rugatulus ants in a large arena over 5h. We approximated the footprint concentration by summing ant visitations for each point in the arena and calculated the speed and local path straightness for each point of the ant trajectories. We counterintuitively find that ants walk slightly faster and straighter in areas with fewer footprints. This is partially explained by the effect that ants who start out from the nest walking straighter move on average further away from the nest, where there are naturally fewer footprints, leading to an apparent relationship between footprint density and straightness However, ants walk slightly faster and straighter off footprints even when controlling for this effect. We tested for klinotaxis by calculating the footprint concentrations perceived by the left and right antennae of ants and found no evidence for a turning-away (nor turning-towards) behavior. Instead, we found noticeable effects of environmental idiosyncrasies on the behavior of ants which are likely to overpower any reactions to pheromones. Our results indicate that search density around an ant colony is affected by several independent processes, including individual differences in movement pattern, local spatial heterogeneities, and ants’ reactions to chemical footprints. The multitude of effects illustrates that non-communicative coordination, individual biases and interactions with the environment might have a greater impact on group search efficiency and exploratory movements than pheromone communication.


Fig S5. 3 .
Fig S5.3.Hunt et al. 2016 'C' version of the main text figures 1 and 2. a), c) Straightness and speed increase with decreasing footprint concentration.b), d) Straightness and speed increases with distance to the nest.Black lines are medians per footprint and nest distance bins, respectively.Gray lines are regressions from the LMMs.Statistical tests were performed on unbinned data.Note that the x-axis of the footprint concentration panels are reversed to be consistent with the 'Distance to the nest' plots, as the highest footprint concentrations are around the nest (i.e.towards the left side on both graphs).More ants walk significantly e) straighter and f) faster on lower footprint concentrations than the opposite (see tab S5.2 for numbers).Histograms are of the slopes (for each track) of the linear models of straightness or speed ~ footprint concentration.Negative values mean ants are walking less straight or slower with increasing footprint concentration.Left inset: example track, colored by the footprint concentration the ant is currently walking over.Black open circle indicates the nest location.Right inset: scatterplot with slope of the linear model of straightness~footprints for that track.

Fig. S5. 12 .
Fig. S5.12.Hunt et al. 2016 'C' version of main text figures 5, 6, and 7. Ants walk a) straighter and b) mostly faster with lower footprint concentration, even when binning by (= controlling for) distance to the nest.Bins are sized to each contain about 9.04x10^4 points.c) Slopes of mean final footprint number of 2x2 mm pixels over the straightness value of the first point created in that pixel, for each nest distance bin.d) Same as c), but with speed on the x-axis.e) slopes of the linear model of (straightness~footprints) over the straightness of the first point in the respective pixel.Negative values indicate ants walking straighter on lower footprint concentrations.f) same as e) for initial speed.Filled bars indicate values significantly different from 0.

Table S5 .
1. Ants move straighter and slower further from the nest (fig S5.3 a-d).LMMs with 'colony' as a random factor.

Table S5 .
4. No significant effect of straightness and speed at the start of the tracks with its mean distance (fig S5.5 e-f).

Fig S5.5. Hunt
et al. 2016 'NC' version of main text figures 3 and 4. a), c), d): Heatmaps of the last 5 min of the experiment.a) ant visitation, binned into pixels of size 1 cm.Points outside the black rectangle

Table S5 .
10.When the first point of a pixel was less straight than 0.6, ants walk straighter with increasing number of footprints, and when the first point of a pixel was straighter than 0.6, ants walk less straight with increasing number of footprints (stats to figS5.12e)

Table S5 .
11.When the first point of a pixel was slower than 9 mm/s, ants walk faster with increasing number of footprints, and when the first point of a pixel was slower than 9 mm/s, ants walk slower with increasing number of footprints (stats to figS5.12f)